As recording means used with a camera, there is a known method including forming a subject image on an imaging device formed of photoelectric conversion devices, such as CCD (charge coupled device) and CMOS (complementary metal oxide semiconductor) elements, converting the amount of light of the subject image into an electric output in the photoelectric conversion device, and recording the electric output.
On the other hand, recent advances in micro-processing technologies have allowed a central processing unit (CPU) to operate faster and a storage medium to have higher packing density than ever, whereby a large amount of image data that has not been handled before can now be processed at high speed. Photoelectric conversion devices have also been packed more densely and reduced in size. The higher packing density allows higher spatial frequency recording, and the size reduction allows reduction in entire camera size.
The higher packing density and size reduction described above, however, disadvantageously narrow the light receiving area of each photoelectric conversion device and increase its susceptibility to noise as its electric output level decreases. To address the problems, the amount of light that reaches each photoelectric conversion device is increased by increasing the aperture diameter of an optical system, and tiny lens elements (what is called a microlens array) are disposed immediately in front of the photoelectric conversion devices. The microlens array advantageously guides a light flux directed to the space between adjacent photoelectric conversion devices to these photoelectric conversion devices but disadvantageously constrains the exit pupil position of a lens system. The reason for this is that when the exit pupil position of the lens system approaches the photoelectric conversion devices, that is, when the principal ray that reaches each of the photoelectric conversion devices forms a large angle with the optical axis of the lens system, off-axis light fluxes directed toward the periphery of the screen are inclined to the optical axis by large angles and hence do not reach photoelectric conversion devices, resulting in an insufficient amount of light.
In general, what is called a standard zoom lens has an angle of view ranging from about 24 to 35 mm on a 35-mm format basis in an wide angle end state in which the focal length of the zoom lens is minimized and an angle of view greater than 50 mm on a 35-mm format basis in a telescopic end state in which the focal length is maximized.
The standard zoom lens has employed a positive, negative, positive, and positive four-group configuration and a positive, negative, positive, negative, and positive five-group configuration in many cases. A positive, negative, positive, and positive four-group zoom lens includes a first lens group having positive power, a second lens group having negative power, a third lens group having positive power, and a fourth lens group having positive power sequentially arranged from the object side. A positive, negative, positive, negative, and positive five-group zoom lens includes a first lens group having positive power, a second lens group having negative power, a third lens group having positive power, a fourth lens group having negative power, and a fifth lens group having positive power sequentially arranged from the object side.
JP-A-2002-323656, JP-A-2003-241092, and JP-A-2007-114432, for example, describe positive, negative, positive, and positive four-group zoom lenses, and JP-A-2009-156891 and JP-A-2010-237453, for example, describe positive, negative, positive, negative, and positive five-group zoom lenses.
In a positive, negative, positive, and positive four-group zoom lens, the second lens group is mainly responsible for magnification changing operation. In this case, increasing the power of the second lens group for size reduction and a higher magnification ratio disadvantageously results in insufficient correction of aberrations produced by the second lens group itself and a difficulty in correcting change in off-axis aberrations that occurs when a lens position setting is changed. In the zoom lenses described in JP-A-2002-323656, JP-A-2003-241092, and JP-A-2007-114432, since the third lens group includes a positive power partial group and a negative power partial group, the combination of the third lens group and the fourth lens group has a power arrangement of a positive power partial group, a negative power partial group, and a positive power partial group. As a result, off-axis light fluxes passing through the first lens group in the wide angle end state shift away from the optical axis, disadvantageously resulting in an increase in the lens diameter of the first lens group. In a positive, negative, positive, negative, and positive power five-group zoom lens, since the combination of the third lens group to the fifth lens group has the same power arrangement, it is also difficult to reduce the lens diameter of the first lens group.
It is therefore desirable to provide a variable focal length lens system that includes a first lens group having a small lens diameter and has a small, lightweight configuration as a whole. It is also desirable to provide an imaging apparatus having the same advantages.